High performance pneumatic rubber tires typically contain rubber treads where traction on the road (skid resistance) is desired.
Such tire treads may contain one or more resins to aid in promoting tread traction.
While the traction enhancement by the resin content may be due to various factors, the melting point (or softening point) of the resin is normally considered important because, as the resin melts and therefore softens, it undergoes a phase transition and its mechanical properties change.
The softening of the resin in the tread rubber composition is seen as promoting a degree of softening of the tread rubber composition and tends to increase the hysteresis of the tread rubber composition, and thereby promote traction of the tread running surface on the road, usually at a rubber temperature substantially equivalent to or slightly above the resin's melting point.
Accordingly, a resin with a lower softening point (melting point) may be desirable as the tire is run from a stationary, resting position to vehicular driving speeds where the temperature of the tread may increase from its stationary ambient temperature (e.g. 23° C.) to a somewhat higher operating temperature.
In such circumstance, a resin with a melting point of about 30° C. might be expected to soften and promote a very hysteretic (predictive internal heat generating) rubber composition at a tread temperature of about 30° C. to about 50° C. and, therefore aid in promoting tire tread traction at such tread temperature.
Thereafter, as the tread temperature increases significantly higher to, for example, 100° C., for example, such resin would be expected to be in a softened or perhaps liquid state and the tread traction would, accordingly, be expected to be affected only to a very limited degree by the aforesaid 30° C. melting point resin as the rubber temperature increases to temperatures significantly higher than 30° C.
Accordingly, the presence in the tread rubber composition of a resin with a considerably higher melting point would be desirable to promote tread traction at the higher tread temperature associated with the higher vehicular speed.
It is apparent that as the resin softens, the cured tread rubber composition containing the softened resin becomes more hysteretic as a result of the softened resin, and therefore predicatively more prone to internal heat generation within the rubber composition. This means that, as the tire tread is being run at higher speeds, the tread rubber composition has a greater tendency to transform internal energy generated within the tread into heat, and results in a significant temperature increase of the rubber composition and a resultant improved traction performance for the tread.
Representative examples of resins which have heretofore been proposed to promote tire tread traction for tread rubber compositions are, for example, hydrocarbon-derived synthetic resins, coumarone-indene resins, rosin, rosin derivatives, dicyclopentadiene based resins such as, for example, dicyclopentadiene/diene resins and polyester phthalate resins.
A significant aspect of this invention is an inventive implementation of a combination of spaced apart melting point (softening point) resins, solution polymerization prepared spaced apart high Tg, high styrene-containing styrene/butadiene elastomers (SBR's) where at least one of such SBR's is a functionalized SBR and reinforcing filler composed of a combination of rubber reinforcing carbon black and precipitated silica, to promote tread traction and a degree of resistance to tread wear over a broad range of operating temperatures.
It is envisioned that the discovery of the beneficial combination of resins, SBR's and carbon black/silica reinforcement provides a significant departure from past practice and enables a high performance tire tread with enhanced traction and handling capability.
In particular a significant contribution of the combination of resins with spaced apart softening points is seen to promote a hysteretic property of the rubber composition over a broad temperature range to thereby promote internal dynamic heat generation within the rubber composition to consequently promote an increase of the rubber temperature of the tire tread itself to thereby promote enhanced traction of the tread over ground over a broad tire tread operating temperature range.
In particular, a significant contribution of the high styrene content of the SBR elastomers is envisioned to promote traction of the tire tread over ground over a broad tire tread operating temperature range.
In particular, a significant contribution of the functionality such SBR(s) is envisioned to promote stability of the elastomer and reinforcing filler network as well as to promote overall durability of the tread, particularly resistance to treadwear.
In particular, a significant contribution of use of a combination of selective carbon black reinforcement together with precipitated silica plus silica coupling agent reinforcement is envisioned to promote a combination of the silica reinforcement effect on wet traction for the tread running surface and of the selective carbon black on treadwear resistance.
A beneficial collective effect is envisioned to promote a maintenance of the resistance to treadwear while achieving traction for the tread running surface.
The carbon black reinforcement is represented by rubber reinforcing carbon black selected for high structure and fine particle size. In particular, the high DBP value for the selected carbon black is indicative of a high structure for the carbon black which is envisioned to promote stability and durability for the tread rubber composition. In particular, the high Iodine value for the selected carbon black is indicative of a fine carbon black particle size which is envisioned to promote traction for the tread running surface and resistance to treadwear for the rubber tire tread.
Historically, it is appreciated that tire treads have heretofore been proposed which rely upon a plurality of resins with spaced apart softening points to promote tread traction (for example see U.S. Pat. Nos. 6,221,990; 6,221,953; 6,242,550; 6,274,685; 6,316,567; and 6,357,499). However, this invention is considered herein to be a significant departure from such practice particularly because of the inclusion of a combination of relatively high styrene containing SBR elastomers with spaced apart Tg's, at least one of which is further required to be a functionalized SBR, as well as reinforcing filler composed of specialized selection of rubber reinforcing carbon black together with precipitated silica and its silica coupling agent.
Historically, it is appreciated that tire treads have heretofore been proposed with various elastomers having spaced apart Tg's for various purposes such as example, U.S. Pat. Nos. 6,465,560 and 5,723,530. However, this invention is considered herein to be a significant departure from such practice particularly because a combination of relatively high styrene containing SBR elastomers is required with spaced apart Tg's, at least one of which is further required to be a functionalized SBR, all of which is required to be used in a tire tread rubber composition which contains at least three resins with spaced apart softening points together with reinforcing filler composed of specialized selection of rubber reinforcing carbon black together with precipitated silica and its silica coupling agent.
In the description of this invention, the terms “rubber compound”, “sulfur-cured rubber compound” or “rubber composition”, “rubber blend” and “compounded rubber” may be interchangeably used to refer to rubber which has been mixed with rubber compounding ingredients. Such terms are well known to those having skill in such art. The term “phr” is used to refer to parts by weight per 100 parts by weight rubber, as is a conventional practice.
A reference to glass transition temperature of an elastomer, or Tg, as referred to herein, as well as a reference to a resin's melting point, represents an inflection point glass transition temperature of the respective elastomer determined by a differential scanning calorimeter (DSC) at a temperature rate of 110° C. per minute by convention procedure well known to those having skill in such art.
A reference to a resin's softening point as referred to herein relates to its softening point determinable by ASTM E28-58T, sometimes referred to as a “Ring and Ball” softening point.